Roaming refers to service that enables a mobile station to continuously use communication service without interruption in service or loss in connectivity when the mobile station moves from the service network of a communication operator, with which it is registered, to the service network of another communication operator.
Roaming service provided in conventional communication networks includes roaming service between 2G CDMA (Code Division Multiple Access) operators, roaming service between GSM (Global System for Mobile Communications) operators, roaming service between 3G WCDMA (Wideband Code Division Multiple Access) operators, roaming service between a GSM operator and a WCDMA operator, and so forth.
When a mobile station roams between circuit switched (CS) networks, such as a 2G CDMA network, a GSM network, and a WCDMA network, there is an interface that makes it possible to provide supplementary services, serviced by a home network, to the corresponding mobile station in a roaming network. Accordingly, a mobile station using roaming service between CS networks can be provided with supplementary services, such as calling line identification presentation/restriction service and call holding/call waiting service, as well as voice service.
In recent years, communication operators have revealed a desire to simultaneously provide VoIP service, serviced by an Internet protocol multimedia subsystem (IMS) network, and voice service, serviced by a CS network, to one dual mode mobile station.
To this end, when a mobile station roams between an IMS network and a CS network, a function is needed to provide not only the voice service, but also supplementary services, serviced by a home network, that is, the IMS network, to the mobile station in the CS network to which the mobile station roams.
Up to now, however, there is no network architecture or interface for providing supplementary services to a mobile station when the mobile station roams between an IMS network and a CS network. Accordingly, there is a problem in that a mobile station roaming from an IMS network to a CS network cannot be provided with or control supplementary services, serviced by the IMS network, in the CS network.